The present invention relates to a method of inspecting pipelines such as gas pipelines and to a pipeline pig for use in such a method.
Pipeline inspection is important in order to investigate the integrity of pipelines and, if possible, enable warning of possible failure to be given well in advance of such failure so that pre-emptive measures can be taken to avoid it or mitigate its results. A particular problem arises with buried gas pipelines where fracture may have dire consequences.
There have been many proposals for pigs and methods that can monitor the structural condition of a pipeline, including pigs which have optical sensors or cameras, pigs with ultrasonic transducers and the like, but the results achieved with such devices are not always satisfactory, at least partly, because the results achieved may not give any indication of surrounding features in the ground which may lead to fracture in due course. FIG. 12 illustrates a buried gas pipeline P which passes over regions of soft S and hard H soil, where conditions vary greatly from those immediately surrounding them. The passage of heavy vehicles such as that shown at V will eventually cause sagging at region S and hogging at region H as indicated in the two small diagrams to each side of the main figure. Fracture is ultimately likely to occur at points marked *.
GB-A-2146772 proposes using a pig to strike the wall of a pipeline to introduce reverberant vibratory signals into the pipeline and then listening to the resultant reverberations and detecting preselected characteristics of the received sound to indicate unsupported sections or points of load or stress.